Paperboard is used to create packages for a variety of consumer products such as pharmaceuticals, home entertainment, health and beauty aids, food, and tobacco products. Insulated cups and folding containers are widely used for serving hot and cold beverages and other food items. Such articles may be made from a variety of materials including polystyrene foam, double-walled containers, and multi-layered paper-based containers such as paperboard containers containing an outer foamed layer. Paper-based containers are often more desirable than containers made from styrene-based materials because paper-based materials are generally more amenable to recycling, are biodegradable and have a surface more acceptable to printing. However, multi-layered and multi-walled paper-based containers are relatively expensive to manufacture compared to polystyrene foam-based articles and often do not exhibit comparable insulative properties. Paperboard containers having an outer foam insulation layer are generally less expensive to produce than double-walled containers, but the outer surface is less compatible with printing.
Print mottle is an undesirable quality in offset printing. Specifically back trap print mottle is observed in coated paperboard and other coated substrates when the print from the previous station comes in contact with the subsequent stations which can range from two additional stations to as many as six or more additional stations. This print mottle can be caused by variety of reasons, including, binder migration during the drying of the coating process, poor basesheet formation and non-uniform coat weight distribution. Print mottle reduction may involve controlling the drying strategies after coating, which may limit the productivity and require additional capital to overcome them. Any method that can reduce the print mottle can be useful in generating an aesthetically appealing product.
A low-density coated paperboard with improved mottle is desirable from an aesthetic and economic perspective. A reduction in paperboard density results in a more economical product requiring less material and energy input to produce an equal area of paperboard. The print characteristics of coated paperboard are dependent on a complex interaction of basesheet structure, coating properties and lay down, and the finishing process of the coated product. In an ideal situation, a well formed basesheet (good formation) is lightly finished before calendering (to minimize densification) and the coating formulation and equipment allow a uniform coating distribution that is then finished to give a smoother surface without much further densification. In practice, this is difficult to achieve, with formation of baseheets being in regimes such that excessive calendering is required to achieve target smoothness levels before coating. Densification of the paperboard is not desirable from a cost of manufacture perspective. Further, excessive densification of the basesheet can contribute to nonuniform binder migration, which could contribute to print mottle. Existing methods of correcting densification of basesheet include 1) multiply machines with bulky fibers, such as BCTMP and other mechanical fibers in the center plies of paperboard, 2) use of extended nip press sections for reducing densification during water removal, and 3) alternate calendering technologies for basestock, including hot soft calendering, hot steel calendering, steam moisturization, shoe nip calendering. These options typically require significant capital and can be economically prohibitive. If the basestock is not finished to target smoothness, higher coat weights need to be used for achieving desirable print quality. While the basestock density may be lower in this case, the coating cost would increase significantly and increase the overall cost and increase the density of the final product.
Therefore, there is a need for a method and an apparatus to reduce the density of coated paperboard with improved or desirable smoothness and print quality.